1. Field of the Invention:
This invention relates in general to structural members and methods of fabricating structural members. More specifically it relates to, but is not limited to, structural members that are used as slabs, and the method of fabricating and fastening these members to other parts of the structure.
2. Description of the Prior Art:
In the construction of slabs for bridges or buildings, there are a wide variety of methods and materials in use. The most commonly used method is to form or construct the slabs in their final position in the structure. Usually this involves placing a slab of concrete over a grid or framework of steel o concrete structural members. In order to conserve materials the steel and concrete are connected at their interface with mechanical connectors so that they act together to resist the applied loads. This method of using materials and structural components that act together is called composite construction.
A wide variety of prefabricated structural members have been developed for use in structural slabs. Most of these are fabricated from portland cement concrete, and may be conventionally reinforced with steel bars or prestressed using prestressing steel developed for this purpose. Many of the prefabricated panels used as slabs in buildings or bridges utilize a cast-in-place portland cement concrete topping over the panels. This cast-in-place topping normally bonds to the precast slab to form a one directional composite slab. An additional reason for using a cast-in-place topping is to provide a smooth and/or level surface on the top of the panels.
There are several problems with prefabricated panels used to construct a floor slab for buildings, or a deck slab for bridges. One of these is the difficulty in obtaining a smooth and/or level surface at the joints between adjacent panels. This difficulty can usually be overcome by using a cast-in-place concrete topping, but this cast-in-place topping greatly increases the time required to construct the slab and place it in service. This time factor is extremely important in the replacement of existing in-service bridge decks. When a cast-in-place topping is not used, the top surface of the slab is usually irregular at the joints between slabs, or the cost of leveling or fastening the slabs is greatly increased due to the time and materials required to level the panels.
Another problem associated with the prefabricated panels currently in use is the difficulty in developing composite action between the panels and the main supporting beam and girders of the structure. This composite action is beneficial since it increases the load carry capacity of existing structures, and reduces the sizes of the structural members in new construction. An additional benefit of composite action is the increase in stiffness of the floor or deck system which in turn reduces the displacements due to loads placed on the slab. A further benefit is a reduction of vibrations in the floor or deck with a corresponding increase in the useful life of the slab.
There are several significant advantages gained in the use of precast slab panels in the construction of bridges and buildings. The most significant of these is the savings in construction time and labor which greatly reduces the cost of the structure. When it is necessary to replace the deck slab of existing in-service bridges, the use of precast panels greatly reduces the time that the bridge must be closed to traffic. With properly designed panels and a well organized construction sequence it is possible to replace portions of the deck slab at night or during weekends when interruption to traffic does not cause a major problem.
Another advantage of precast panels is the high quality of concrete that can be produced in the controlled conditions of the fabrication plants. Quality of concrete is affected by the mixing, transporting, placing and curing of the material in its plastic state. It is well known in the construction industry that concrete cast-in-place under field conditions is generally lower in quality and strength than a similar concrete cast in a fabricating plant. It is also understood that the concrete at the bottom of a structural member is more dense and durable than the concrete cast on the top surface. Therefore it is advantageous to cast panels used in slabs in the inverted position.
The object of the present invention is to provide an improved method of constructing slabs for buildings and bridges. In particular it is the object of the present invention to provide precast panels with a durable concrete surface that will act with partially embedded steel structural members t transfer loads to the main beams of the structure, and to also provide a method of connection to the main beams or girders that will produce composite action between these beams or girders and the precast panels. It is also an object of the present invention to provide a method that is low in cost and results in superior structural performance and reduced maintenance.